1. Field of the Invention
This invention relates to transistor back-gate voltage control technology to realize reduced power consumption of semiconductor integrated circuits.
2. Description of the Related Art
Japanese Patent Laid-open No. 9-83335 discloses technology in which a VLSI (Very Large Scale Integration) circuit is divided into a plurality of circuit blocks, switching transistors are positioned between each circuit block and at least either one of a common power supply line and a common ground line, and when a circuit block is in a standby state, the back-gate voltage is controlled such that the threshold voltage of the switching transistor rises, and the switching transistor is turned off. By using this technology, leakage currents (turn-off currents) in the standby state can be decreased, so that power consumption can be reduced.
However, in the above-described technology of the prior art, the back-gate voltage of switching transistors is controlled for the entire circuit on a chip or for each comparatively large circuit block, such as the CPU core or coprocessor, so that the switching transistors are not used to reduce leakage currents of a plurality of finer grain circuits. There is a problem that leakage currents from the sub-circuits during operating state sum up to large amount of current. Further, the larger scale of the circuit is to be controlled, the more time it takes for the back-gate voltage to be settled during switching, so that there is the problem that delays occur in transitions from the standby state to the operating state. Also, in the above-described technology of the prior art, because the power of circuit blocks is occasionally shut down to reduce leakage current, the circuit blocks cannot hold the states. Hence in case of the sequential circuit block, every time a transition from the standby state to the operating state takes place, the initial or current state of the sequential circuits shall be resumed through the sequence of procedure and or by the additional circuit, and then there are the problems that the circuit scale tends to increase and that transitions from the standby state to the operating state become slower.